No one is 100% certain cause we've never managed to make it for longer than a few milliseconds but if you look at a periodic table you'll notice that hydrogen sits right at the top of the alkali metals group. That means that theoretically it should share a lot of their properties.
The idea is that if you cool hydrogen enough and subject it to huge pressure it will turn into a metallic form. This form is known to exist on Jupiter and Saturn. What makes it really interesting is that some scientists believe that once made it may remain a metal after the pressure is removed (in the same way that diamond doesn't turn back into graphite the second the pressure is removed).
Metallic hydrogen would be about as strong as aluminium but would weigh 1/3 as much. It would also be useful as a fuel source (simply turning it back into a gas releases energy. Once you have the gas you can burn it) and it may also be useful in fusion applications as one of the main problems with fusion (getting the attoms close together) is already halfway solved.
Best of all some people think it would also be a room temperature superconductor
